The present invention relates to a lighting ballast, in particular, to an electronic dimming ballast having a front end having an input coupled to an alternating current (AC) supply and having an output supplying a direct current (DC) bus that feeds a back end powering a lighting load, for example, a gas discharge lamp load such as a fluorescent lamp or other lamp load.
There is an increasing need to maximize the efficiency of electronic ballasts. In particular, in electronic dimming ballasts, the total energy consumed is the combination of the energy consumed by the electronic circuitry of the ballast and the energy consumed by the lighting load. In particular, as the lighting load is dimmed to lower levels, the energy consumed by the circuitry of the ballast comprises a greater proportion of the total energy consumed. The proportion of energy consumed by the ballast is typically greatest at the lowest dimming level. It is desirable, therefore, to reduce the amount of energy consumed by the ballast, particularly at low lighting levels.
As an example, a customer specification may call for a high-end input power usage of 35 watts, for example, at full lamp intensity. This may correspond, for example, to approximately 28 watts dissipated in the lamp load and 7 watts in the ballast.
At the low dimming level, corresponding to say, for example, a 10% dimming level, the input power to the ballast may be specified at approximately 10.5 watts or approximately 30% of the high end input power. Typical prior art electronic ballasts may consume approximately 5 watts of input power at the low end dimming level in the ballast. At the high end, the ballast may consume approximately 7 watts, or about ⅕ of the input power. At the low end, the ballast consumption of about 5 watts is about half of the input to the ballast. It is desirable to reduce the amount of power consumed by the ballast, particularly at the low end dimming level when the ballast consumes a greater proportion of the available input energy.
Some electronic dimming ballasts have an active front end for the purpose of reducing total harmonic distortion (THD) in the input line current and raising the power factor. Boost converters using switching transistors are often used for this purpose. However, the switching transistor typically has switching losses associated with its operation. These losses tend to be constant across the dimming range of the ballast. These switching losses are a fairly insignificant proportion of the total power consumed by the ballast and the load when the load is at high end. This is typically viewed as being acceptable in light of the advantages in improved THD and power factor. However, at low end, the improvements in THD and power factor are not as important because the line current drawn by the ballast and the load is much lower. Accordingly, the switching losses in the boost converter comprise a greater proportion of the power consumed by the ballast and the load, without the attendant improvements in THD and power factor.